To formalize the discussion, we call each group of nodes an abstract
node. Thus, we say that an explicit route is a specification of a
set of abstract nodes to be traversed. If an abstract node consists
of only one node, we refer to it as a simple abstract node.

It should say:

To formalize the discussion, we call each group of nodes an abstract
node. Thus, we say that an explicit route is a specification of a
sequence of abstract nodes to be traversed. If an abstract node
consists of only one node, we refer to it as a simple abstract node.

Notes:

s/set/sequence

A set implies ordering of abstract nodes is NOT important.
A sequence implies ordering of abstract nodes IS important.

In the rest of RFC 3209, this distinction is maintained, but not
in this paragraph.